Add Diagnostics section to settings: version, browser engine, log view

For triaging webview rendering issues without devtools: settings panel
now shows app version, the webview user agent, and the application log
(pre-wrap, 80ch, scrollable, refreshable). New endpoints /api/version,
/api/log; client-side JS errors are captured globally and posted to
/api/client-log (client-errors.log in the log dir).

frontend/src/main.ts

@@ -6,6 +6,38 @@ import { installKeyboardNavigation } from "./composables/useKeyboardNavigation"
 import { installGamepadNavigation } from "./composables/useGamepadNavigation"
 import { installInputModalityTracking } from "./composables/useInputModality"
 
+function postClientError(payload: {
+  message: string
+  stack: string | null
+  source: string | null
+}) {
+  fetch("/api/client-log", {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(payload),
+  }).catch(() => {})
+}
+
+function installErrorCapture() {
+  window.addEventListener("error", (event) => {
+    const source =
+      event.filename != null ? `${event.filename}:${event.lineno ?? 0}:${event.colno ?? 0}` : null
+    postClientError({
+      message: event.message || String(event.error ?? "Unknown error"),
+      stack: event.error?.stack ?? null,
+      source,
+    })
+  })
+  window.addEventListener("unhandledrejection", (event) => {
+    const reason = event.reason
+    postClientError({
+      message: reason instanceof Error ? reason.message : `Unhandled rejection: ${String(reason)}`,
+      stack: reason instanceof Error ? (reason.stack ?? null) : null,
+      source: "unhandledrejection",
+    })
+  })
+}
+
 function installReloadShortcut() {
   document.addEventListener(
     "keydown",
@@ -27,6 +59,7 @@ installInputModalityTracking()
 installKeyboardNavigation()
 installGamepadNavigation()
 installReloadShortcut()
+installErrorCapture()
 
 // Unregister any legacy service workers — MediaHive no longer uses a PWA/SW.
 if ("serviceWorker" in navigator) {